Generally, for the exploration and extraction of sub-bottom resources, boreholes are formed in the seabed using drilling equipment mounted on a platform of a stationary or movable structure.
FIG. 1 shows the construction of a conventional drilling system, and the procedure of drilling a borehole will be described with reference to FIG. 1.
A drill pipe 21 having a drill bit 22 on a lower tip thereof is mounted to a top drive 20 installed in a derrick 10. Here, the top drive rotates the drill pipe, and is coupled to a travelling block 30, which is supported by a crown block 40 installed on an upper portion of the derrick so as to move vertically, such that the top drive can move vertically along with the travelling block. Thus, when the drill pipe rotates while moving down, with cooperation of the top drive and the crown block, the borehole is formed in the seabed, and as the depth of the bore hole to be drilled increases, the length of the drill pipe is extended depending on the increased depth by additionally assembling a unit piece to the drill pipe.
When the borehole has been formed to a certain depth, a casing pipe 50 is inserted into and cemented with the borehole to prevent the collapse of the borehole.
Next, the borehole is further dilled to a certain depth using a smaller diameter drill bit, and another casing pipe, a diameter of which is smaller than that of the previously inserted casing pipe, is inserted into and cemented with the borehole.
The formation of the borehole and the coupling of the casing pipe into the borehole are repeated so as to form a borehole having a desired depth. To prevent a sudden gush of pressurized gas, water or oil in the seabed through the borehole in the formation of the borehole, a blow out preventer (BOP) 60 is installed on an upper portion of the borehole (or the casing pipe) in such a manner as to be coupled to a platform 70 by a riser 61.
Installation of the casing pipe or the riser is carried out by assembling a plurality of unit pieces each having a certain length into a single assembly, and installing the assembly into the borehole or on the BOP.
The formation and installation of the assembly is carried out by an activity arranged in the derrick, wherein the activity is composed of the crown block 30, the travelling block 40, the top drive 20, and a rotary table 80.
However, the conventional drilling system is of a single-activity type in which one activity is provided in the derrick, so that, when one of drilling, assembly and installation of the casing pipes, and assembly and installation of the risers is being performed by the activity, another process cannot be carried out in parallel with the ongoing process.
That is, when the drill pipe is installed on the top drive and is drilling the borehole, the casing pipes or the risers cannot be assembled. In this case, after the borehole is completely drilled until the formed borehole has a desired depth, the drill bit and the drill pipe should be separated from the top drive, and then the casing pipes or the risers are to be assembled and installed. This degrades working efficiency and extends working time.
Also, in the case of the single activity type drilling system, when the BOP is to be separated from a currently-drilled borehole in order to form another borehole, the riser and the BOP are first separated and lifted from the borehole (or the casing pipe) using the activity, and the riser and the BOP are stored in a predetermined storage after completely disassembled. This should be done because, if the BOP assembly (including the BOP and the riser) separated from the borehole is left on the activity as it is, other work cannot be done. Thus, for reuse, the disassembled BOP assembly should be re-assembled, inevitably consuming much time.